Single screw extruder method and apparatus

ABSTRACT

An extruder screw having a plurality of plasticating sections sufficient for plasticating polymeric materials of different bulk density.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of application Ser. No. 08/184,510 filed Jan. 19,1994 now abandoned.

FIELD OF THE INVENTION

This invention relates to methods and apparatus useful for theplastication of polymeric material.

BACKGROUND OF THE INVENTION

Methods and apparatus for the plastication of two or more polymericmaterials having different bulk density have been discovered.

SUMMARY OF THE INVENTION

In one aspect this invention involves an extruder screw comprising (a) aplasticating section having a compression ratio in the range of about1.2 to about 5, and (b) a plasticating section having a differentcompression ratio in the range of about 1.2 to about 5.

In another aspect this invention involves, in an extruder forplasticating a mixture of two or more polymeric materials of differentbulk density, a screw comprising (a) a first section in which thematerials are compacted to an extent that the mixture has a bulk densitybetween the highest and lowest of the respective bulk densities of thematerials comprising the mixture, and a portion of the materials areplasticated, and (b) a second section in which any materials notplasticated in the first section are plasticated.

In a further aspect this invention involves a method of plasticating amixture of two or more polymeric materials of different bulk densitycomprising the steps of (a) compacting a first polymeric material to theextent that its bulk density is approximately equal to that of a secondpolymeric material, and (b) further compacting the mixture of polymericmaterials until all materials are plasticated.

In a yet another aspect this invention involves an extruder screw forthe plastication of two or more polymeric materials having differentbulk density, comprising (a) a plasticating section having a compressionratio sufficient to plasticate a polymeric material having a bulkdensity of less than 0.6 g/cm³, and a plasticating section having acompression ratio sufficient to plasticate a polymeric material having abulk density of 0.6 g/cm³ or more.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a cross-sectional view of a plastication section of anextruder screw.

DETAILED DESCRIPTION OF THE INVENTION

A single screw extruder is commonly used instrument for the plasticationof polymeric material. Polymeric material is useful for the fabricationof a variety of molded or shaped articles. Plastication refers to thesoftening of polymeric material to such an extent that it flows freelyand will assume any shape. In the case of polymeric material which iscrystalline, plastication is synonymous with melting. In the case ofpolymeric material which is amorphous, plastication occurs at about theglass transition temperature ("T_(g) ").

Polymeric material may exist in a variety of different physical formssuch as powder, beads, pellets, flakes, chips, fibers and strips. Apolymeric material has a density, which is constant and is an inherentproperty of the material regardless of the shape of the physical form inwhich it exists. However, a polymeric material also has a bulk density,and this property is variable and is determined by the shape of thephysical form in which it exists. Density may be defined as the mass ofa continuous media of material per unit volume, whereas bulk density ofa polymeric material may be defined as the mass of a non-continuousmedia of the material divided by the volume occupied by that mass.

An extruder for use in processing polymeric material is typicallycomposed of a screw, a cylinder having one or more raised ridges (a"flight") helically disposed thereabout, which screw rotates within anannular cylinder, or barrel. The surface of the screw above which theflight(s) are raised is the root of the screw. A hopper directspolymeric material through an opening in the barrel into contact withthe screw and into the space between the flight(s) and the interior wallof the barrel of the extruder.

The screw typically has an initial or feed section which begins theprocess of conveying the solid polymeric material forward within thebarrel of the extruder, the direction of travel of the polymericmaterial as it is transported away from the hopper by the screw beingconsidered the "downstream" direction. The feed section of the screw istypically followed, with or without other intervening sections, by atransition or melting section in which plastication of the polymericmaterial occurs. Plastication of the polymeric material occurs as aresult of the combined effect of heat produced by heater bands mountedon the outside of the extruder barrel and the shearing forces to whichthe screw subjects the polymeric material causing friction between theinternal wall of the extruder barrel and the polymeric material. Themelting section of the screw is typically followed, again with orwithout other intervening sections, by a metering section whichfunctions to pump the plasticated material, as extrudate, out throughthe downstream end of the extruder which is typically a die or someother form of restricted orifice.

Polymeric materials having different bulk densities behave differentlyin the melting, or plastication, section of an extruder specificallybecause of the different bulk densities. When polymeric material isplasticated in an extruder, the plastication mechanism involvescompacting the material to obtain frictional contact of the materialwith the interior wall of the barrel. Materials of higher bulk densitycannot be compacted as easily or readily as those of lower bulk density,and conditions which are appropriate for the compaction of one materialare often not appropriate for another material or for a mixturecontaining that other material. The amount of difference in the bulkdensities of two materials which may cause different compaction behavioris typically at least 20 percent (determined as (ρ₁ -ρ₂)/100/ρ₂, whereρ₁ is greater than ρ₂) may be at least 50 percent, is often at least 100percent, and may on occasion be at least 1000 percent. A first steptoward designing plastication conditions appropriate for polymericmaterials having different bulk densities is to classify those materialsaccording to the effect of bulk density on compaction behavior. It hasbeen found that materials having a bulk density of less than 0.6 g/cm³,and more particularly in the range of about 0.1 g/cm³ to about 0.5g/cm³, may be more easily and readily compacted than those materialshaving a bulk density of 0.6 g/cm³ or more, especially those in therange of about 0.7 g/cm³ to about 0.8 g/cm³.

Knowing the respective bulk densities of different polymeric materialsto be plasticated together makes it possible to provide separateplastication conditions in the extruder, each suitable for a differentmaterial. This is done by constructing an extruder screw with two ormore different plastication sections. The plastication section of anextruder screw typically has a frusto conical shape with thecircumference of the cone increasing in the downstream direction. Aspolymeric material passes through a plastication section of an extruder,the space between the root of the screw and the interior wall of thebarrel (the "channel depth") decreases, and the polymeric material iscompressed. The compression ratio of a plastication section of a screwis determined by a ratio of the channel depth at the top of the frustumto the channel depth at the base of the cone (base of the frustum). Thisis shown in FIG. 1 as the ratio of H₀ /H₁. The compression rate of aplastication section of a screw is determined by a relationship derivedfrom the right triangle formed by a line intersecting the circumferenceof the top of the frustum and parallel to the longitudinal axis of thescrew, the intersection of such line with the base of the cone (base ofthe frustum) and the root of the screw. This relationship is thecompression ratio divided by the quantity L/D where L is the distancefrom the top to the base of the cone frustum and D is the diameter ofthe annulus of the barrel. These elements are also shown in FIG. 1.

It has been found that when two or more plastication sections areconstructed in a screw for plastication of a mixture of materials havingdifferent bulk densities, a sufficient compression ratio for each suchsection may be in the range of about 1.2 to about 5. The compressionratio for a section in which a material having a bulk density of 0.6g/cm³ or more is to be plasticated may preferably be in the range ofabout 1.2 to about 3.3, and the compression ratio for a section in whicha material having a bulk density of less than 0.6 g/cm³ is to beplasticated may preferably be in the range of about 3.3 to about 5. Asufficient compression rate for each such plastication section may be inthe range of about 0.01 to about 15, preferably about 0.05 to about13.7. The compression rate is selected such that, for a giventemperature and pressure within the extruder, a greater length isallowed for the plastication section of the screw for those materialswhich are relatively more difficult to plasticate because of, forexample, a higher bulk density and/or a higher melting or softeningtemperature. The plastication sections may be constructed in the screwin any order such that one section is followed downstream by another,the downstream section having a higher or lower compression ratio and/ora higher or lower compression rate than the upstream section. One ormore such plastication sections may, but need not be, immediatelyadjacent.

When a mixture of polymeric materials having different .bulk densitiesis plasticated by means of a screw having two or more separateplastication sections, all, a first polymeric material may be compactedto the extent that its bulk density is approximately equal to that of asecond polymeric material, and/or the materials may be compacted to anextent that the mixture has a bulk density between the highest andlowest of the respective bulk densities of the materials comprising themixture. While this occurs, a portion of each material is carrieddownstream in the first section by the screw as a bed of unplasticatedsolids. However, the volume of such solids bed is progressivelydecreased in the first section by compression against the interior wallof the barrel as it is carried toward the base of the frusto conicalshaped plastication section. Progressively decreasing the volume of thesolids bed has the result of keeping it in contact with the interiorwall of the barrel as a progressively larger portion of the one of thematerials becomes plasticated. In a second section, all of the remainingunplasticated material is plasticated. A good indication that allmaterial has been plasticated is that the extruder screw maintains anessentially constant output of extruded material and that the amperageof the motor which powers the extruder remains essentially constant.

Virtually any polymeric material which can be melted or heat softenedmay be plasticated in a mixture with another such polymeric material bythe methods and apparatus of this invention. The following are exemplaryof polymers and copolymers, or alloys or blends of two or more thereof,which are suitable for such plastication, but this listing is notintended to be exhaustive or to limit the scope of this invention:polyacetal, including that which is formed by the bond opening andpolymerization of the carbonyl group of an aldehyde to give a --(--CH₂--O--)-- repeating unit, as well as the reaction products of polyols andaldehydes;

polyacrylamide;

polyacrylate;

polyacrylonitrile;

polyamide;

polyarylate

poly(arylene sulfide), including that which is prepared by the reactionof p-dichlorobenzene with Na₂ S in a polar organic solvent to give a--(--_(p) Ar--S--)-- repeating unit;

azo polymers, including those which are prepared by the polymerizationof an azobutyronitrile with a diamine or diol to give a--(--R--N═N--R--)-- repeating unit or those prepared by polymerizationof monomers containing an azo side group in addition to a polymerizablefunctionality to give a --[--R(N═N--R')--]-- repeating unit;

polybenzimidazole, including that which is prepared by condensation ofaromatic tetraamino compounds with dicarboxylic acids;

polycarbonate, including copolymers thereof;

polyester, including copolymers thereof;

poly(ethylene oxide);

polyimidazole, including that which is prepared by polymerization ofvinylimidazole monomer;

polyimide, including that which is prepared by condensation ofbifunctional carboxylic acid anhydrides with a diamine to give a--[--C(O)--N--R--C(O)--]-- repeating unit;

poly(methyl methacrylate);

polyolefin, including copolymers thereof;

poly(phenylene ether), including that which is prepared by the oxidativecoupling polymerization of a phenol to give a --(--_(p) Ar--O--)--repeating unit;

polyphosphazine, including that which is prepared by the polymerizationof the cyclic trimer produced by the reaction of phosphorouspentachloride and ammonium chloride to give a --[--N═P(R₂)--]--repeating unit;

poly(propylene oxide); polyquinoxaline, including that which is preparedby the solution polymerization of aromatic bis(_(o) diamine) andbis(glyoxal hydrate);

polysilane;

polysiloxane;

polystyrene, including copolymers thereof;

polysulfone;

polyurea;

polyurethane; and

vinyl polymers, including poly(vinyl acetate), poly(vinyl alcohol),poly(vinyl amide), poly(vinyl chloride), and poly(vinyl ether),including copolymers of each;

where, in any of the above formulae, R and R' are organic (e.g. C₁ -C₂₀)radicals and Ar is an aromatic organic (e.g. C₆ -C₁₀) radical.

This invention is also applicable to copolymers formed from two or moremonomers or co-monomers, such copolymers including but not being limitedto:

acrylonitrile/butadiene/styrene copolymer,

acrylonitrile/EPDM/styrene copolymer where EPDM isethylene/propylene/diene rubber,

acrylonitrile/styrene/alkyl acrylate copolymer,

poly(ester/carbonate) copolymer,

ethylene/carbon monoxide copolymer,

ethylene vinyl acetate copolymer,

methyl methacrylate/butadiene/styrene/acrylonitrile copolymer,

phenyl maleimide/acrylonitrile/butadiene/styrene copolymer,

styrene/acrylonitrile copolymer, and

styrene/maleic anhydride copolymer;

as well as to blends and alloys of two or more polymers and/orcopolymers. Examples of such blends being:

acrylonitrile/butadiene/styrene copolymer blended with polycarbonate,polyamide, polyester, or polyurethane;

polyacetal blended with polyurethane;

polycarbonate blended with polyamide, polyester, orstyrene/acrylonitrile copolymer; and

polyphenylene ether blended with (i) polyamide and a vinylaromatic/conjugated diene di- or tri-block copolymer; (ii) polyester; or(iii) polystyrene to which can be grafted a vinyl aromatic/conjugateddiene di- or tri-block copolymer.

What is claimed is:
 1. An extruder screw comprising two or moreplasticating sections, each with a frustum and flights forming achannel, characterized in that a first, upstream, plasticating sectionhas a compression ratio in the range of about 1.2 to about 5, and isadjacent to a second, downstream, plasticating section which has adifferent compression ratio in the range of about 1.2 to about 5, whichcompression ratios are equal to the ratio of the channel depth at thetop of the frustum to the channel depth at the base of the frustum. 2.The extruder screw of claim 1 wherein the second plasticating sectionhas a lower compression ratio than the first plasticating section. 3.The extruder screw of claim 1 wherein the second plasticating sectionhas a higher compression ratio than the first plasticating section. 4.The extruder screw of claim 1 wherein each of said first and secondplasticating sections has the same or different compression rates in therange of about 0.01 to about 15, which compression rates are equal tothe compression ratio divided by the quantity L/D where L is thedistance from the top to the base of the cone frustum and D is thediameter of the annulus of the barrel.
 5. The extruder screw of claim 4wherein the second plasticating section has a lower compression ratiothan the first plasticating section.
 6. The extruder screw of claim 4wherein the second plasticating section has a higher compression ratiothan the first plasticating section.
 7. The extruder screw of claim 4wherein each of said first and second plasticating sections has the sameor different compression rates in the range of about 0.05 to about 13.7.8. The extruder screw of claim 1 which maintains a constant output ofextruded polymeric material.
 9. The extruder screw of claim 1 whereinone of said first and second plasticating sections has a compressionratio sufficient to plasticate a polymeric material having a bulkdensity of less than about 0.6 g/cm³, and the other of said first andsecond plasticating sections has a compression ratio sufficient toplasticate a polymeric material having a bulk density of about 0.6 g/cm³or more.
 10. The extruder screw of claim 1 wherein one of said first andsecond plasticating sections has a compression ratio sufficient toplasticate a polymeric material having a bulk density of about 0.1 toabout 0.5 g/cm³.
 11. The extruder screw of claim 1 wherein one of saidfirst and second plasticating sections has a compression ratiosufficient to plasticate a polymeric material having a bulk density ofabout 0.7 to about 0.8 g/cm³.